Microporous polyethylene membranes having low fusing temperatures

ABSTRACT

A polyethylene microporous film which is composed of a linear copolymeric polyethylene having a melt index (MI) of less than 0.1 and a propylene unit content of 0.1 to 4 mol % and which has a fuse temperature of less than 136° C. A polyethylene microporous film which is composed of a mixture of a linear copolymeric polyethylene having a melt index (MI) of less than 0.1 and a propylene unit content of 0.1 to 4 mol % and a high density polyethylene having a comonomer unit content of less than 0.1 mol %, said mixture having a weight average molecular weight of 250,000 to 700,000 and a propylene unit content of 0.1 to 4 mol %, and which has a fuse temperature of less than 136° C.

This application claims the benefit under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP96/03560 which has an Internationalfiling date of Dec. 5, 1996 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreferences.

TECHNICAL FIELD

This invention relates to a polyethylene microporous film, a separatorcomposed thereof and a battery in which the same is used as a batteryseparator.

BACKGROUND ART

Polyethylene microporous films are used in micro filtration membranes,battery separators, condenser separators and the like. Among these uses,the use as a battery separator, particularly a separator for lithium ioncell, requires that the polyethylene microporous film exerts, inaddition to general physical properties such as mechanical strength andfilm permeability, a so-called “fuse effect” such that when the innerpart of the battery is overheated, the separator fuses to form a film,which covers the electrode, thereby cutting off the electric current tosecure the safety of battery.

In the case of a polyethylene microporous film, it is known that thefuse temperature, namely the temperature at which the fuse effect isexerted, is about 130 to 150° C. Even when the inner part of a batteryis overheated by any cause, the microporous film fuses upon reaching thefuse temperature to form a film with which the electrode is covered, sothat the passage of ions is cut off and the cell reaction is stopped.However, when the temperature elevation is very rapid, the temperaturein the inner part of battery is further elevated even after thepolyethylene microporous film has fused, and consequently the film isbroken, which causes a short in the battery in some cases, so that thedevelopment of a polyethylene microporous film having a lower fusetemperature has become a problem.

For example, JP-A-5-25,305 and JP-A-2-21,559 disclose a method forlowering the fuse temperature of a film by blending into an ultrahighmolecular weight polyethylene (UHMWPE) a branched, low-densitypolyethylene (LDPE) or a linear, low-density polyethylene (LLDPE).According to this method, the fuse temperature of the film can beexpected to be lowered to some extent. On the other hand, however, themelt index (MI) is 0.1 to 100 which is very high as compared with UHMWPEand HDPE, so that there have been problems such as the mechanicalstrength and permeability of film are deteriorated, and when the amountof LDPE and LLDPE added is increased the film is not made porous.Moreover, the above method has had a problem in film productivity, suchas at least one hour being required for dissolving the polymer byheating because UHMWPE having a low dissolvability is used as theessential component.

The object of this invention is to solve the above-mentioned problemsand provide a polyethylene microporous film which is excellent inmechanical strength, permeability and productivity and has a low fusetemperature.

The present inventors have examined a linear copolymeric polyethylenehaving a much higher molecular weight than LLDPE which has been used inthe prior art. As a result, it has been found that a linear copolymericpolyethylene can be made porous even when used alone and a polyethylenemicroporous film produced therefrom has a high strength and a low fusetemperature, which compares quite well with the prior art. However, sucha film has not had a sufficient permeability for using as a separator ascompared with a film produced from HDPE having a high molecular weight.

Therefore, a further examination has been made on the kind of acomonomer to be introduced as a monomeric unit into the linearcopolymeric polyethylene, and the present inventors have surprisinglyfound that a microporous film composed of a linear copolymericpolyethylene in which propylene is used as the comonomer (referred tohereinafter as the C3 copolymer in some cases) is smaller in shrinkagethan a microporous film composed of a copolymer in which butene-1 isused as the comonomer (referred to hereinafter as the C4 copolymer insome cases) and a microporous film composed of a copolymer in whichother α-olefins are used, and has a sufficient permeability for using aseparator.

The reason why the C3 copolymer specifically imparts a high porosity tothe film has not been clarified, but it is considered that the methylgroup which is the side chain of the C3 copolymer is easily incorporatedinto the crystals of the polymer as compared with an ethyl group andbutyl group, so that it could be possible that the C3 copolymer has acrystalline structure close to HDPE in spite of being a copolymer, andexhibits a higher permeability than copolymers in which other comonomersare used such as the C4 copolymer.

Further, the present inventors have made extensive research to achieve ahigher permeability and have consequently found that when the C3copolymer is not used alone but used in admixture with a high densitypolyethylene, a higher permeability is obtained simultaneously with thesame fuse temperature, and based on this knowledge, they have succeededin producing a battery having more improved discharge characteristicsand safety.

DISCLOSURE OF INVENTION

The first mode of this invention is a polyethylene microporous filmwhich is composed of a linear copolymeric polyethylene having a meltindex of less than 0.1 and a propylene unit content of 0.1 to 4 mol %and which has a fuse temperature of less than 136° C.

The second mode of this invention is a polyethylene microporous filmwhich is composed of a mixture of a linear copolymeric polyethylenehaving a melt index of less than 0.1 and a propylene unit content of 0.1to 4 mol % and a high density polyethylene having a comonomer unitcontent of less than 0.1 mol %, the above mixture having a weightaverage molecular weight of 250,000 to 700,000 and a propylene unitcontent of 0.1 to 4 mol % and which has a fuse temperature of less than136° C.

The third mode of this invention is a separator composed of theabove-mentioned polyethylene microporous film.

The fourth mode of this invention is a battery in which theabove-mentioned separator is used as a battery separator.

BEST MODE FOR CARRYING OUT THE INVENTION

The melt indexes (MI) of the C3 copolymer and the mixture of the C3copolymer and a high density polyethylene are less than 0.1, preferablyless than 0.07 and more preferably less than 0.05. When the MI is 0.1 ormore, it is difficult to make the film porous. As the C3 copolymerhaving such an MI and the above mixture, those having a weight averagemolecular weight of 200,000 to 4,000,000, preferably 250,000 to 700,000,and more preferably 250,000 to 500,000 are mentioned.

When the C3 copolymer is used alone, the weight average molecular weightthereof is 200,000 to 700,000, preferably 250,000 to 600,00, and morepreferably 250,000 to 500,000.

The propylene unit content of the C3 copolymer is 0.1 to 4 mol %,preferably 0.2 to 3 mol %, and more preferably 0.5 to 2 mol %, based onthe ethylene unit. When the propylene unit content is less than 0.1 mol% based on the ethylene unit, the lowering of the fuse temperature isinsufficient, and when it exceeds 4 mol %, the crystallinity is loweredtoo much and it becomes difficult to make the film porous.

The density of the C3 copolymer is 0.85 to 0.97, preferably 0.90 to0.96, and more preferably 0.92 to 0.95.

The C3 copolymer used in this invention can be produced by various knownmethods. For example, it can be produced by polymerization using achromium compound-supported catalyst or magnesium compound-containingZiegler catalyst as disclosed in JP-B-1-12,777.

Furthermore, in this invention, by mixing the above C3 copolymer with ahigh density polyethylene having a comonomer unit content of less than0.1 mol % (referred to hereinafter as HDPE in some cases), the relationbetween permeability and fuse temperature can be improved more than whenthe C3 copolymer is used alone. The reason therefor has not beclarified; however, it is considered that when a mixture having alowered comonomer concentration is used, which is prepared by uniformlydispersing the C3 copolymer having a high comonomer concentration in ahigh density polyethylene having a high porousness to dilute it, therelation between porousness and fuse effect can be better improved thanwhen a C3 copolymer having the same comonomer concentration as the abovemixture is used alone.

The weight average molecular weight of HDPE which can be mixed is100,000 to 4,000,000, preferably 200,000 to 1,000,000, and morepreferably 250,000 to 700,000.

The proportion of the C3 copolymer contained in the mixture is 1% to100%, preferably 5% to 90%, and more preferably 10% to 80%. When theproportion of the C3 copolymer is less than 1%, it is difficult toobtain a sufficiently low fuse temperature.

The weight average molecular weight of the mixture is 250,000 to700,000, preferably 250,000 to 700,000, and more preferably 250,000 to500,000.

The propylene unit content of the C3 copolymer can be selected from therange of from 0.1 to 4.0 mol %; however, it is preferable to set, inanticipation of dilution with HDPE, a higher propylene unit content thanwhen it is used alone.

An explanation is made below of the present process for producing thepolyethylene microporous film.

The polyethylene microporous film is produced by dissolving polyethyleneat a temperature not lower than its melting point in a solvent called aplasticizer, cooling the resulting solution to a temperature not higherthan its crystallization temperature to produce a polymer gel,subjecting the polymer gel to film-formation (film-forming step),stretching the film obtained (the stretching step) and thereafterremoving the plasticizer (the plasticizer-removing step).

The plasticizer referred to herein means an organic compound capable offorming a uniform solution of polyethylene at a temperature not higherthan the boiling point of the plasticizer, and as specific examplesthereof, there are mentioned decalin, xylene, dioctyl phthalate, dibutylphthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol,diphenyl ether, n-decane, n-dodecane, paraffin oil and the like. Amongthem, paraffin oil, dioctyl phthalate and decalin are preferred. Theproportion of the plasticizer in the polymer gel is not particularlylimited; however, it is 20% to 90%, preferably 50% to 70%. When theproportion of the plasticizer is 20% or less, it is difficult to obtaina microporous film having an appropriate porosity, and when it is 90% ormore, the viscosity of a hot solution is lowered, which makes thecontinuous formation of a sheet difficult.

Hereinafter, the process for producing the polyethylene microporous filmis divided into the film-forming step, the stretching step and theplasticizer-removing step and each is explained separately.

[Film-forming step]

The film-forming step is not particularly limited; however, for example,a sheet having a thickness of several tens of μm to several mm can beformed by feeding a linear copolymeric polyethylene powder and aplasticizer to an extruder, kneading the two at a temperature of about200° C., and thereafter casting the mixture from a usual hanger coat dieonto a cooling roll.

In carrying out this invention, an ultrahigh molecular weightpolyethylene is not used as the essential component unlike priortechniques, so that no particular heating and dissolving equipment isrequired, a homogenous sheet can be prepared very simply by only feedinga polyethylene and a plasticizer to an extruder.

[Stretching step]

Subsequently, the sheet obtained is stretched at least monoaxially toform a stretched film. The stretching method is not particularlylimited; however, a tentering method, a rolling method, a calenderingmethod and the like can be used. Among them, simultaneous, biaxialstretching by a tentering method is preferred. The stretchingtemperature is a temperature ranging from room temperature to themelting point of the polymer gel, preferably 80 to 130° C, and morepreferably 100 to 125° C. The stretching ratio is 4 to 400 times,preferably 8 to 200 times, and more preferably 16 to 100 times, as anareal ratio. When the stretching ratio is not more than 4 times, thestrength as a separator is insufficient, and when it is not less than400 times, not only is the stretching difficult, but also such adverseeffects as the porosity of the microporous film obtained is low, and thelike, tend to result.

[Plasticizer-removing step]

Subsequently, a microporous film is obtained by removing the plasticizerfrom the stretched film. The method of removing the plasticizer is notparticularly limited. For example, when paraffin oil or dioctylphthalate is used as the plasticizer, it is sufficient to extract thesame with an organic solvent such as methylene chloride, methyl ethylketone or the like. However, the solvent can be sufficiently removed byheating and drying the microporous film obtained at a temperature nothigher than the fuse temperature thereof. Moreover, when, for example, alow-boiling point compound such as decalin or the like is used as theplasticizer, the solvent can be sufficiently removed by only heating anddrying the microporous film at a temperature not higher than the fusetemperature thereof. In any case, it is preferable to remove theplasticizer while restraining the film by fixing the film or the like.

In order to improve permeability and enhance dimensional stability, thepolyethylene microporous film obtained by the above-mentioned productionprocess is, if necessary, subjected to heat treatment at a temperaturenot higher than the fuse temperature.

[Physical properties]

The thickness of the microporous film is 1 to 500 μm, preferably 10 to200 μm, and more preferably 15 to 50 μm. When the film thickness is lessthan 1 μm, the mechanical strength of the film is not sufficient, andwhen the film thickness is more than 500 μm, it becomes difficult tominiaturize and save weight in the battery.

The porosity, which is used as a measure of porousness, and the gastransmission rate, which is used as a measure of permeability, areaffected by the conditions in the plasticizer-removing step, so that inthis invention, they are evaluated by numerical values obtained when theremoval of plasticizer is effected by extraction with methylene chlorideat ordinary temperature in the non-restraint state.

The porosity in this invention is 20 to 80%, preferably 35 to 50%. Whenthe porosity is less than 20%, the permeability is not sufficient andwhen it is more than 80%, no sufficient mechanical strength is obtained.

The gas transmission rate in this invention is 10 sec to 6,000 sec,preferably 50 sec to 4,000 sec, and more preferably 100 sec to 2,000sec. When the gas transmission rate is more than 6,000 sec thepermeability is not sufficient, and when the gas transmission rate isless than 10 sec the pore diameter is too large, so that the above gastransmission rate is not desirable.

The diameters of pores in the microporous film are 0.001 to 0.3 μm,preferably 0.005 to 0.1 μm, and more preferably 0.01 to 0.05 μm. Whenthe pore diameter is less than 0.001 μm, the permeability is notsufficient and when the pore diameter is more than 0.3 μm, cutting-offof electric current due to the fuse effect occurs late and besides ashort circuit due to precipitated dendrites and degraded activesubstances is feared, so that a film having such fine pores is notsuited for use as a battery separator.

This invention is further explained in more detail below by Examples, inwhich “part” is “part by weight” in all appearances.

The methods of testing the characteristics shown in the Examples are asfollows:

(1) Film thickness

Determined by use of a dial gauge (Ozaki Sei-sakusho: PEACOCK No. 25).

(2) Porosity

A sample of 20-cm square was cut from a microporous film and the volumeand weight thereof were determined, after which the porosity wascalculated from the results obtained using the following equation:Porosity (%)=100×[volume (cm³)−weight (g)/0.95]/volume.

(3) Sticking strength

Using a KES-G5 handy compressive tester manufactured by Katotech, asticking test was conducted using a pin having a tip curvature radius of0.5 mm at a sticking speed of 2 mm/sec and the maximum sticking load wasdetermined as the maximum sticking strength (g). Also, a value obtainedby multiplying the sticking strength by a value of 25 (μm)/filmthickness (μm) was determined as a 25 μ-reduced sticking strength.

(4) Gas transmission rate

Measured by a Gurley densometer according to JIS-P-8117. Also, a valueobtained by multiplying the gas transmission rate by a value of 25(μm)/film thickness (μm) was determined as a 25-μ reduced gastransmission rate.

(5) Pore diameter

1) SEM method: Measured by a scanning type electron microscope.

2) Gas transmission method: The pore diameter of a microporous film canbe calculated from porosity and gas transmission rate using thefollowing equation by assuming the Kundsen flow in the determination ofgas transmission rate:

Pore diameter (μm)=189×τ²/{porosity (%)×25-μ reduced gas transmissionrate (sec)} wherein the flexing rate τ of pore was made 2.0 as to allmicroporous films.

(6) Comonomer unit content

The comonomer unit content (mol %) is determined by multiplying by 100the quotient obtained by dividing the mole-reduced amount (A) of theintegral value of signal strengths due to the comonomer unit by the sumof (A) and the mole-reduced amount (B) of the integral value of signalstrengths due to ethylene unit in the ¹³C-NMR spectrum.

For example, when propylene is used as the comonomer, in the followingstructure model:

wherein I₁, I_(1′), I₂, I₃, I_(a), I_(b), I_(c) I_(m) and I_(M)represent the respective signal intensities resulting from thecorresponding carbons in the ¹³C-NMR spectrum, Comonomer unit content(mol %)=(A)/[(A)+(B)]×100 wherein

(A)=(I_(1′)+I_(m)+I_(a)/2)/3

(B)=(I₁+I₂+I₃+I_(M)+I_(a)/2+I_(b)+I_(C))/2.

Therefore, the signal numbers I₁, I₂ and I₃ resulting from the terminalcarbons are neglected and the above equation is arranged to obtain thefollowing equation: Comonomer unit content (mol%)=I_(m)/[I_(m)+(I_(M)+5I_(m))/2]×100.

(7) Melt index

The melt index measured based on JIS K-7210 at a temperature of 190° C.under a load of 2.16 kg and was made MI. Incidentally, when MI becomesabout less than 0.01 which is enough smaller than 0.1, variation tendsto be caused in the measurement of MI, so that when the comparison ofmelt indexes is effected in more detail, HMI obtained by measurementunder a load of 21.6 kg can be used together with the usually used MI(load: 2.16 kg).

(8) Fuse temperature

An electrolyte solution prepared by adding lithium borofluoride to amixed solvent of propylene carbonate and butyrolactone (volumeratio=1:1) so that the concentration became 1.0M was used to impregnatetherewith a polyethylene microporous film cut to a diameter of 16 mm,and this film was nipped by two sheets of a nickel-made electrode undera pressure of 20 kg/cm² and the impedance variation when the temperaturewas elevated from room temperature at a rate of 20° C./min was measuredunder the conditions of 1 V and 1 kHz. In this measurement, thetemperature at which the impedance reached 1,000 Ω was determined as thefuse temperature.

(9) Overcharge test

A lithium ion cell was produced by using as a positive electrode a sheetobtained by mixing LiCoO₂ used as a positive active material, graphiteand acetylene black used as conducting agents and fluororubber used as abinding agent, at a weight ratio of LiCoO₂ : graphite acetylene black :fluororubber=88:7.5:2.5:2, mixing dimethylformamide with the resultingmixture to prepare a paste, coating this paste on an Al foil and thendrying the same; as a negative electrode a sheet obtained by mixingdimethylformamide with a mixture of needle cokes : fluororubber=95:5weight ratio to prepare a paste, coating this paste on a Cu foil andthen drying the same; and an electrolyte solution prepared by addinglithium borofluoride to a mixed solvent of propylene carbonate andbutyrolactone (volume ratio=1:1) so that the concentration became 1.0M.This cell was charged at 4.2 V for 5 hours, and then furtherover-charged at constant current. The internal temperature of the cellwas elevated by the overcharge and when the temperature reached the fusetemperature of a sample the electric current was cut off. Samples onwhich current return was not caused for at least 10 minutes thereafterere evaluated as “O”. Incidentally, the present test is n acceleratedtest, so that the test was conducted in such a state that a safetydevice with which the actual battery was provided such as PTC device orthe like was removed.

EXAMPLE 1 (THIS INVENTION)

Using a batch system melt kneading machine (Toyo Seiki: Laboplastomill),40 parts of a C3 copolymer (density: 0.934, propylene unit content: 0.7mol %) having an MI of 0.016, an HMI of 0.60 and a weight averagemolecular weight of 430,000, 60 parts of paraffin oil (Matsumura SekiyuKenkyusho: P350P) and 0.5 part of a thermal stabilizer (Ciba Geigy:Irganox 245) were kneaded at 200° C. at 50 rpm for 10 minutes. Theresulting mixture was molded by a heated press at 200° C. and thereaftercooled by a water-cooled press to prepare an original sheet having athickness of 1,000 μm. This original sheet was stretched 6×6 times at115° C. by use of a simultaneous biaxial stretching machine (ToyoSeiki), and thereafter, the paraffin oil was removed by extraction withmethylene chloride. Physical properties of the film obtained are shownin Table 1.

EXAMPLE 2 (this invention)

In the same manner as in Example 1, except that a C3 copolymer (density:0.929, propylene unit content: 1.6 mol %) having an MI of 0.017, an MHIof 0.42 and a weight average molecular weight of 420,000 was used, apolyethylene microporous film was prepared. Physical properties of thefilm obtained are shown in Table 1.

EXAMPLE 3 (this invention)

In the same manner as in Example 1, except that a C3 copolymer (density:0.931, propylene unit content: 1.9 mol %, provided measured by an IRspectrum) having an MI of less than 0.01, an HMI of 1.58 and a weightaverage molecular weight of 330,000 was used, a polyethylene microporousfilm was prepared. Physical properties of the film obtained are shown inTable 1.

EXAMPLE 4 (this invention)

In the same manner as in Example 1, except that a C3 copolymer (density:0.930, propylene unit content: 1.8 mol %, provided measured by an IRspectrum) having an MI of less than 0.01, an HMI of 0.72 and a weightaverage molecular weight of 420,000 was used, a polyethylene microporousfilm was prepared. Physical properties of the film obtained are shown inTable 1.

EXAMPLE 5 (this invention)

In the same manner as in Example 1, except that a C3 copolymer (density:0.933, propylene unit content: 1.6 mol %, provided measured by an IRspectrum) having an MI of less than 0.01, an HMI of 0.65 and a weightaverage molecular weight of 480,000 was used, a polyethylene microporousfilm was prepared. Physical properties of the film obtained are shown inTable 1.

EXAMPLE 6 (this invention)

In the same manner as in Example 1, except that a C3 copolymer (density:0.929, propylene unit content: 1.9 mol %, provided measured by an IRspectrum) having an MI of less than 0.01, an HMI of 0.46 and a weightaverage molecular weight of 520,000, a polyethylene microporous film wasprepared. Physical properties of the film obtained are shown in Table 1.

EXAMPLE 7 (comparison)

In the same manner as in Example 1, except that a C4 copolymer (density:0.928, butene unit content: 1.3 mol %) having an MI of 0.013, an HMI of0.54 and a weight average molecular weight of 400,000 was used, apolyethylene microporous film was prepared. Physical properties of thefilm obtained are shown in Table 1.

EXAMPLE 8 (comparison)

In the same manner as in Example 1, except that 40 parts of a highdensity polyethylene (density 0.956, comonomer unit content: 0.0 mol %)having an MI of 0.025, an HMI of 2.33 and a weight average molecularweight of 250,000 and 60 parts of paraffin oil (Matsumura SekiyuKenkyusho: P350P) were used, a polyethylene microporous film wasprepared. Physical properties of the film obtained are shown in Table 1.

EXAMPLE 9 (comparison)

The same method as in Example 1, except that LLDPE (density: 0.917)having an MI of 0.4 was used and the extraction was conducted in therestraint state, was tried; however, the appearance of the film wastransparent and no microporous film was obtained.

EXAMPLE 10 (comparison)

The same method as in Example 1, except that 40 parts of LDPE (density:0.917) having an MI of 0.3 and 60 parts of paraffin oil (MatsumuraSekiyu: P350P) were used, was tried; however, the original sheet had toolow a strength to be stretched.

EXAMPLE 11 (this invention)

A polyethylene microporous film was prepared in the same manner as inExample 1, except that 6 parts of a C3 copolymer (density: 0.929,propylene unit content: 1.6 mol %, provided measured by an IR spectrum)having an MI of 0.017 and a weight average molecular weight of 420,000,34 parts of a high density polyethylene (density: 0.956, comonomer unitcontent: 0.0 mol %) having a weight average molecular weight of 250,000and 60 parts of paraffin oil (Matsumura Sekiyu Seisakusho: P350P) wereused and the stretching temperature was 120° C. Using the film obtained,the melt index (HMI) and propylene unit content of its polyethylene weremeasured. As a result, the HMI was 1.43 and the propylene unit contentwas 0.5 mol % (measured by an IR spectrum). Physical properties of thefilm obtained are shown in Table 2.

EXAMPLE 12 (this invention)

A polyethylene microporous film was prepared in the same manner as inExample 1, except that 12 parts of a C3 copolymer (density: 0.929,propylene unit content: 1.6 mol %, provided measured by an IR spectrum)having an MI of 0.017 and a weight average molecular weight of 420,000,28 parts of a high density polyethylene (density: 0.956, comonomer unitcontent: 0.0 mol %) having a weight average molecular weight of 250,000and 60 parts of paraffin oil (Matsumura Sekiyu Seisakusho: P350P) wereused and the stretching temperature was 120° C. Using the film obtained,the melt index (HMI) and propylene unit content of its polyethylene weremeasured. As a result, the HMI was 1.46 and the propylene unit contentwas 0.8 mol % (measured by an IR spectrum). Physical properties of thefilm obtained are shown in Table 2.

EXAMPLE 13 (this invention)

A polyethylene microporous film was prepared in the same manner as inExample 1, except that 20 parts of a C3 copolymer (density: 0.929,propylene unit content: 1.6 mol %, provided measured by an IR spectrum)having an MI of 0.017 and a weight average molecular weight of 420,000,20 parts of a high density polyethylene (density: 0.956, comonomer unitcontent: 0.0 mol %) having a weight average molecular weight of 250,000and 60 parts of a paraffin oil (Matsumura Sekiyu Seisakusho: P350P) wereused and the stretching temperature was 120° C. Using the film obtained,the melt index (HMI) and propylene unit content of its polyethylene weremeasured. As a result, the HMI was 0.96 and the propylene unit contentwas 0.9 mol % (measured by an IR spectrum). Physical properties of thefilm obtained are shown in Table 2.

EXAMPLE 14 (this invention)

A polyethylene microporous film was prepared in the same manner as inExample 1, except that 20 parts of a C3 copolymer (density: 0.925,propylene unit content: 1.9 mol %, provided measured by an IR spectrum)having a weight average molecular weight of 830,000, 20 parts of a highdensity polyethylene (density: 0.956, comonomer unit content: 0.0 mol %)having a weight average molecular weight of 250,000 and 60 parts of aparaffin oil (Matsumura Sekiyu Seisakusho: P350P) were used. Using thefilm obtained, the melt index and propylene unit content of itspolyethylene were measured. As a result, the MI was less than 0.01, theHMI was 1.26 and the propylene unit content was 1.6 mol % (measured byan IR spectrum). Physical properties of the film obtained are shown inTable 2.

EXAMPLE 15 (this invention)

A polyethylene microporous film was prepared in the same manner as inExample 1, except that 20 parts of a C3 copolymer (density: 0.925,propylene unit content: 1.9 mol %, provided measured by an IR spectrum)having a weight average molecular weight of 830,000, 20 parts of a highdensity polyethylene (density: 0.962, comonomer unit content: 0.0 mol %)having a weight average molecular weight of 140,000 and 60 parts of aparaffin oil (Matsumura Sekiyu Seisakusho: P350P) were used. Using thefilm obtained, the melt index and propylene unit content of itspolyethylene were measured. As a result, the MI was 0.019, the HMI was3.26 and the propylene unit content was 1.8 mol % (measured by an IRspectrum). Physical properties of the film obtained are shown in Table2.

EXAMPLE 16 (comparison)

The same method as in Example 1, except that 20 parts of a high densitypolyethylene (density: 0.956, comonomer unit content: 0.0 mol %) havinga weight average molecular weight of 250,000, 20 parts of LLDPE(density: 0.917) having an MI of 0.4 and 60 parts of a paraffin oil(Matsumura Sekiyu Seisakusho: P350P) were used and the stretchingtemperature was 120° C., was tried; however, the film obtained wastransparent and no microporous film was obtained.

EXAMPLE 17 (comparison)

The same method as in Example 1, except that 20 parts of a high densitypolyethylene (density: 0.956, comonomer unit content: 0.0 mol %) havinga weight average molecular weight of 250,000, 20 parts of LDPE (density:0.917) having an MI of 0.3 and 60 parts of paraffin oil (MatsumuraSekiyu Seisakusho: P350P) were used and the stretching temperature was120° C., was tried; however, the film obtained was transparent and nomicroporous film was obtained.

EXAMPLE 18 (this invention)

Using a 35-mm twin-screw extruder, there were kneaded 20 parts of a C3copolymer (density: 0.929, propylene unit content: 1.6 mol %, providedmeasured by an IR spectrum) having an MI of 0.017 and a weight averagemolecular weight of 420,000, 20 parts of a high density polyethylene(density: 0.956, comonomer unit content: 0.0 mol %) having a weightaverage molecular weight of 250,000, 60 parts of paraffin oil (MatsumuraSekiyu Seisakusho: P350P) and 0.1 part of an antioxidant (Ciba Geigy:Irganox 245) at 200° C. and then cast onto a cooling roll adjusted to atemperature of 30° C. from a hanger coat die having a lip space of 1,550μm to prepare an original sheet having a thickness of 1,550 μm. Thisoriginal sheet was continuously stretched 7×7 times at 120° C. using asimultaneous biaxial stretching machine and then subjected to extractionwith methylene chloride to remove the paraffin oil, thereby obtaining acontinuous film. Using the film obtained, the melt index (HMI) andpropylene unit content of its polyethylene were measured. As a result,the HMI was 0.96 and the propylene unit content was 0.9 mol % (measuredby an IR spectrum). Using this, a spiral lithium cell was prepared andsubjected to an overcharge test. The results are shown in Table 3.

EXAMPLE 19 (comparison)

In the same manner as in Example 18, except that 40 parts of a highdensity polyethylene (density: 0.956, comonomer unit content: 0.0 mol %)having an MI of 0.025, an HMI of 2.33 and a weight average molecularweight of 250,000 was used, a continuous film was obtained. The resultsare shown in Table 3.

TABLE 1 Example 1 Example 2 Example 3 (this (this (this invention)invention) invention) Film thickness (μm) 23 21 27 Porosity (%) 39 27 31Pore diameter SEM method 0.01 0.01 — (μm) Gas transmission 0.015 0.0070.005 method Sticking strength (g/25 μ) 720 700 450 Gas transmissionrate (sec/25μ) 1260 4000 4730 Fuse temperature (° C.) 135 127 126Remarks C3 C3 C3 Example 4 Example 5 Example 6 Example 7 Example 8Example 9 Example 10 (this (this (this (compari- (compari- (compari-(compari- invention) invention) invention) son) son) son) son) Filmthickness (μm) 24 24 24 25 22 — — Porosity (%) 30 31 27 16 45 — 12 Porediameter SEM method — — — 0.01 0.01 — — (μm) Gas transmission 0.0050.008 0.008 0.002 0.037 method Sticking strength (g/25 μ) 560 580 610640 520 — 410 Gas transmission rate (sec/25μ) 5500 3030 4100 21000 450 —— Fuse temperature (° C.) 129 133 129 120 137 — — Remarks C3 C3 C3 C4HDPE HDPE HDPE LLDPE 50% LDPE 50%

TABLE 2 Example 11 Example 12 Example 13 (this invention) (thisinvention) (this invention) Film thickness (μm) 23 24 18 Porosity (%) 4341 36 Pore diameter SEM method 0.01 0.01 0.01 (μm) Gas transmission0.036 0.032 0.028 method Sticking strength (g/25 μ) 510 480 530 Gastransmission rate (sec/25μ) 490 570 760 Fuse temperature (° C.) 135 134132 Remarks HDPE HDPE HDPE C3 15% C3 30% C3 50% Example 14 Example 15Example 16 Example 17 (this invention) (this invention) (comparison)(comparison) Film thickness (μm) 27 25 — 12 Porosity (%) 32 34 — — Porediameter SEM method — — — — (μm) Gas transmission 0.009 0.010 — — methodSticking strength (g/25 μ) 760 670 — 410 Gas transmission rate (sec/25μ)2500 2200 — — Fuse temperature (° C.) 133 133 — — Remarks HDPE HDPE HDPEHDPE C3 50% C3 50% LLDPE 50% LDPE 50%

TABLE 3 Example 8 Example 19 (this invention) (comparison) Filmthickness (μm) 26 29 Porosity (%) 40 48 Pore diameter (μm) 0.01 SEMmethod 0.01 0.033 Gas transmission 0.021 method Sticking strength (g)560 620 Gas transmission rate 920 480 (sec) Fuse temperature (° C.) 132137 Overcharge test 2A ∘ x         3A ∘ x Remarks HDPE HDPE C3 50%

Industrial Applicability

The polyethylene microporous film of this invention is excellent inmechanical strength, transparency and productivity and simultaneouslyhas a low fuse temperature, and therefore, particularly when it is usedas a separator for battery such as lithium ion cell or the like, abattery having a high reliability can be prepared.

What is claimed is:
 1. A polyethylene microporous film which is composedof a linear copolymeric polyethylene having a melt index (MI) of lessthan 0.1, and a propylene unit content of 0.1 to 4 mol %, wherein thepolyethylene microporous film has a fuse temperature of less than 136°C., a thickness of 1 to 500 μm, a porosity of 20 to 80%, a pore diameterof 0.001 to 0.3 μm a sticking strength of 450 to 760 g/25 μ and a gastransmission rate of 10 to 6,000 sec.
 2. The polyethylene microporousfilm according to claim 1, wherein the above linear copolymericpolyethylene has a weight average molecular weight of 250,000 to700,000.
 3. The polyethylene microporous film according to claim 2,wherein the above linear copolymeric polyethylene has a weight averagemolecular weight of 250,000 to 500,000.
 4. A polyethylene microporousfilm which is composed of a mixture of a linear copolymeric polyethylenehaving a melt index (MI) of less than 0.1 and a propylene unit contentof 0.1 to 4 mol % and a high density polyethylene having a comonomerunit content of less than 0.1%, said mixture having a melt index (MI) ofless than 0.1 and containing 1.0% to 100% of said linear copolymericpolyethylene and which has a fuse temperature of less than 136° C. 5.The polyethylene microporous film according to claim 4, wherein theabove mixture has a weight average molecular weight of 250,000 to700,000.
 6. The polyethylene microporous film according to claim 5,wherein the above mixture has a weight average molecular weight of250,000 to 500,000.
 7. A separator composed of the polyethylenemicroporous film according to claim
 1. 8. A battery in which thepolyethylene microporous film according to claim 1 is used as a batteryseparator.